Simple eyepieces for a microscope comprise a plurality of lenses or lens components with a positive refractive power. Some additionally contain a field lens with a positive refractive power, in order to favorably influence the imaging of the field stop or the diameter of the eye lens. The direction and curvature of the field curvature of an eyepiece are quantitatively described by its Petzval sum SP, where EQU SP=f/n
(with f=focal length of the eyepiece; n=average refractive index).
In such an eyepiece, this sum is positive and large, so that in the case of corrected astigmatism the field curvature is also large.
To offer a viewer at the microscope a flattened image, the micro-objective must have a field curvature opposite to the field curvature of the eyepiece. For this purpose, it is known in the aberration correction of microscopes to work with a compensating model in which micro-objectives and eyepieces with matching astigmatism are combined. Such a compensating model is described in a paper by W. Klein in "Jahrbuch fur Optik" [optical systems annual], year 1977, pages 95-127.
In this case, the astigmatism is not fully corrected in mutually assigned, so-called compensating micro-objectives and so-called compensating eyepieces, but is respectively set to a specific value. This respectively set value of the astigmatism is referred to hereafter as nominal astigmatism. A compensating micro-objective produces on the basis of its nominal astigmatism in the intermediate image a field curvature which is flattened by the assigned compensating eyepiece on the basis of its nominal astigmatism. For this purpose, the nominal astigmatism of the compensating eyepiece is set such that it is equal in magnitude and opposite in operational sign to the nominal astigmatism of the compensating micro-objective.
Although the invention relates to a compensating optical system, from here on the compensating eyepieces are referred to as eyepieces and the compensating micro-objectives are referred to as micro-objectives for the sake of simplicity.
Since the requirements for the micro-objectives--for example even better correction in the VIS/UV range of the spectrum with simultaneously high UV transparency, even better apochromatic correction, and even longer working distance, etc.--have constantly increased, it is becoming ever more difficult to implement such micro-objectives technically and commercially.
To implement such novel, highly corrected micro-objectives, their nominal astigmatism had to be fixed at half the previously customary value. Therefore, to retain the compensation, eyepieces with a halving of their nominal astigmatism and their nominal field curvature, corresponding to the oppositely equal value, are required for the new, halved nominal astigmatism of the novel micro-objectives.
Corresponding to the lower nominal astigmatism in comparison with the previous eyepieces, these eyepieces must be distinguished by a very small Petzval sum SP. The actual magnitude of the Petzval sum depends on the quality of the toleranced residual aberrations. For a highly corrected eyepiece with 10.times. magnification and a field of view number FVN=25, the Petzval sum SP is to be, for example, approximately 0.015 in order to provide the required new value of the nominal astigmatism. The aberrations are then closely toleranced in accordance with the highly corrected micro-objectives. In the case of an eye accommodated to infinity, the eye lens is set to 0 dioptrics. For this setting of the eye lens, the maximum transilluminated, so-called free lens diameter must not exceed 30 mm in order to ensure binocular viewing into the tube, even for small distance-between-eyes positions, on the basis of the outside dimensions of the eyepieces.
The prior art includes eyepieces with a small Petzval sum which are distinguished by a field lens and a strongly refractive negative lens in the so-called eye lens part, i.e. the lens group between the eye and the real intermediate image in the eyepiece. Such eyepieces are mentioned below and--after corresponding conversion--are examined for their possible use in the present compensating system.
For instance, U.S. Pat. No. 5,255,121 specifies such an eyepiece which has a Petzval sum greater than 0.015. It is disadvantageous, however, that coma, zonal astigmatism, pupil difference and distortions (&gt;3%) are outside the desired range. It has two single lenses as the field lens part, with a field lens factor beta'=1.053. It requires a total of seven lenses, which make the eyepiece expensive. If the free lens diameter is scaled to the field of view number FVN=25, it is greater than 30 mm.
The eyepiece from U.S. Pat. No. 3,867,018 has six lenses, of which a thin, plano-convex lens is used as the field lens with a field lens factor beta'=0.767. At 0.007, the Petzval sum is too small for the desired compensating optical system. The coma and astigmatic distortion are also very great, as is the longitudinal chromatic aberration of the pupil. What is more, the free lens diameter, scaled to the field of view number FVN=25, is greater than 30 mm.
DE 39 25 246 C2 specifies in one embodiment an eyepiece with a cemented field lens component with a field lens factor beta'=1.047. It comprises only six lenses. What is disadvantageous is the poor image correction with strong coma, excessive pupil difference and a distortion of over 3%. The free lens diameter, scaled to a field of view number FVN=25, is significantly greater than 30 mm. What is more, the Petzval sum for this field of view number is greater than the desired value of 0.015.
JP 07063996 A describes an eyepiece in which the negative lens is arranged in the second position in the eye lens part. The negative lens is implemented as a lens component with a negative refractive power, optionally with or without a further added negative lens. The field lens is a lens component in the form of a meniscus which is convex with respect to the micro-objective. The field lens factor is equal to 0.996. The eyepiece is well corrected and the free lens diameter is less than 30 mm. However, its Petzval sum is equal to 0.019. What is more, it requires seven or eight lenses, which makes the eyepiece expensive.
Although the eyepieces mentioned mostly have a low Petzval sum, it is not appropriate for the specific requirements which the existing compensating system has to meet. What is more, they either have deficiencies in their imaging performance and/or are made with too many lenses. The free lens diameter is also too great in the case of most known eyepieces.